Re: [PATCH] sched: fix warning in sched_setaffinity

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Nov 21, 2024 at 04:09:16PM -0800, Josh Don wrote:
> On Mon, Nov 11, 2024 at 10:27 AM Josh Don <joshdon@google.com> wrote:
> >
> > Commit 8f9ea86fdf99b added some logic to sched_setaffinity that included
> > a WARN when a per-task affinity assignment races with a cpuset update.
> >
> > Specifically, we can have a race where a cpuset update results in the
> > task affinity no longer being a subset of the cpuset. That's fine; we
> > have a fallback to instead use the cpuset mask. However, we have a WARN
> > set up that will trigger if the cpuset mask has no overlap at all with
> > the requested task affinity. This shouldn't be a warning condition; its
> > trivial to create this condition.
> >
> > Reproduced the warning by the following setup:
> >
> > - $PID inside a cpuset cgroup
> > - another thread repeatedly switching the cpuset cpus from 1-2 to just 1
> > - another thread repeatedly setting the $PID affinity (via taskset) to 2
> >
> > Fixes: 8f9ea86fdf99b ("sched: Always preserve the user requested cpumask")
> > Signed-off-by: Josh Don <joshdon@google.com>
> > Acked-by: Waiman Long <longman@redhat.com>
> > Tested-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
> > Acked-and-tested-by: Vincent Guittot <vincent.guittot@linaro.org>
> > ---
> >  kernel/sched/syscalls.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> > index 4fae3cf25a3a..3a88f7c0cb69 100644
> > --- a/kernel/sched/syscalls.c
> > +++ b/kernel/sched/syscalls.c
> > @@ -1321,7 +1321,7 @@ int __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
> >                         bool empty = !cpumask_and(new_mask, new_mask,
> >                                                   ctx->user_mask);
> >
> > -                       if (WARN_ON_ONCE(empty))
> > +                       if (empty)
> >                                 cpumask_copy(new_mask, cpus_allowed);
> >                 }
> >                 __set_cpus_allowed_ptr(p, ctx);
> > --
> > 2.46.0.469.g59c65b2a67-goog
> >
> 
> Hey,
> 
> I wanted to bump this one last time. It's a pretty simple change that
> already has ACK's from 3 other folks, but has seemed to fall through
> the cracks [1].

Got it, I'll stick it in tip/sched/urgent post -rc1 or so.